I found the rules on Force Fields in Ascension, but have not seen them in the main RT rule book. Am I missing them in there?

I think that the only other place they appear is in the Inquisitor's Handbook for DH, although the rules are a little different from the Ascension ones.

DW

Rules for Force Fields in Rogue Trader can be found in Into the Storm . They work the same as the ones in Ascension, but there's a slightly different range of fields available to obtain.

So the rules are in a book I am not able to get right now. How different are they if you can tell me? What are the ranges in comparison?

The rules are pretty much identical. It's just the force fields you can get are slightly different.